Rotatory instability of the knee permitting abnormal external rotation of the tibia on the femur is the result of forced abduction of the flexed knee and external rotation of the tibia. The basic lesion which is present in all patients is a tear in the medial capsular ligament. Occasionally, this may be present as an isolated lesion but more commonly there are associated lesions of the tibial collateral and anterior cruciate ligaments. A test for rotatory instability of the knee, based on the modification of the test for the anterior drawer sign has been presented. The essential feature of this test is the demonstration of a lateral shift of the longitudinal axis of rotation in the horizontal plane following rupture of the medial capsular ligament. It is stressed that rupture of the anterior cruciate ligament need not be present when the test is positive for rotatory instability, although it is most markedly positive when the anterior cruciate is torn.